Re:new born fry and air!

#5017
Pete Giwojna
Guest

Dear David:

Yes, sir, a newborn seahorse’s first instinct is typically to head to the surface to fill its swim bladder. Pelagic seahorse fry, which undergo a free-swimming or planktonic stage of development, tend to remain near the surface afterwards. Benthic seahorse fry, however, typically make a quick trip to the surface to inflate their swim bladders and then immediately settle down to a bottom-dwelling existence. If you are not present to witness the birth of pelagic seahorse fry, your first indication that the stallion has given birth is often a writhing mass of newborn sea horses, hopelessly tangled together at the top of the tank.

This dangerous situation develops because a newborn’s first instincts are to head to the surface to fill its air bladder and then to anchor itself to something solid. In the vastness of the ocean this is not a problem, since strong currents rapidly disperse the young, but in the confines of an aquarium, the first hitching post it finds will very likely be the tail or snout of one of its siblings (Giwojna, Jan. 1997). The same mistake is apt to be repeated by the rest of the pelagic fry, as they cluster at the surface, until the entire spawn is snarled together tail-to-tail, head-to-tail, tail-to-snout and so on (Giwojna, Jan. 1997). This is a very common experience when raising seahorses such as Hippocampus erectus, H. reidi, and H. ingens, which produce large broods of pelagic fry. (When that’s the case, and you discover a living logjam of newborn pelagic seahorses tangled together at the surface, it’s generally safe to assume that they have already filled their swim bladders, and the youngsters can be immediately dispersed to kriesel or pseudokreisel nurseries to help overcome their surface-hugging tendencies while they undergo further rearing, as discussed in more detail below.)

Once locked together in this Gordian knot, the babies are unable to feed and the aquarist must intervene to avert a disaster. The best way to deal with this potentially lethal logjam is to gentle disentangle the baby sea horses, one by one, if necessary (Giwojna, Jan. 1997). This will give you an opportunity to examine each of the newborns individually, and you should begin culling the fry as you separate them from the tangle.

It is at this point that it is appropriate to carefully transfer the pelagic fry to a specially designed nursery tank (kriesels, pseudokreisels, in-tank nurseries, etc.) that will keep the fry away from the surface. They will already have inflated their swim bladders, which they do almost immediately, and from that point on they will do best if protected against their tendency to congregate at the surface. In other words, the danger is no longer that they will become sliders, but rather that they may ingest too much air and become floaters.

That’s why seahorse fry that undergo an extended pelagic phase, during which they drift freely with the plankton, are much more troublesome to raise than benthic seahorse fry, which orient to the substrate and seek out hitching posts straightaway. The pelagic fry are difficult because the surface huggers tend to gulp air and suffer fatal buoyancy problems, and may even become entrapped by surface tension. As a result, most hobbyists find that mortality is very high during the pelagic phase.

Special precautions must be taken to circumvent the surface-hugging behavior of pelagic fry and the problems this presents, and above all, to prevent them from accidentally ingesting air. The planktonic seahorse fry feed at the surface where their prey tends to congregate, drawn to the light, and all too often the newborns take in air along with their food and cannot expel this air. When this happens, it upsets the fry’s equilibrium, and they will float sideways on the surface of the water. Upon close examination of these floaters, a bubble of trapped air can be spotted just below the head (Tracy Warland, pers. com.).

Sadly, such fry are doomed. Once air has been ingested, there is nothing the aquarist can do to save the delicate babies. Therefore, once the newborns have had an initial opportunity to fill their swim bladders, pelagic fry must be kept away from the surface.

The upshot for the seahorse breeder is that as a rule you needn’t be concerned about pelagic seahorse fry inflating their air bladders initially, other then making sure they will have access to the surface at birth, since they take care of that on their own and do so almost immediately after being born. When the hobbyist notices that birth has occurred and the newborn pelagic fry or congregating at the surface en masse, the focus of his attention at this point should immediately shift to transferring them into nurseries that will counteract their surface-hugging tendencies thereafter.

This is best accomplished by scooping the newborns up in a small measuring cup or something similar along with a little water. It’s important that the newborns aren’t exposed to the air during the transfers. Or a plastic turkey baster works well for delicately sucking up the fry when transferring them, providing you cut off a bit from the tip of the baster to enlarge the opening at the tip.

The reason exposure to the air is a serious problem for newborn seahorses but of no concern at all for older seahorses is due to the presence or absence of the pneumatic duct. Allow me to elaborate.

In seahorses as in other fish, the gas bladder arises as a simple pouch or outgrowth from the foregut (Evans, 1998). In newborn seahorses, this connection with the gut is retained as an open tube, called the pneumatic duct, and seahorse fry gulp air at the surface to fill their gas bladder initially. There is only a short window of opportunity to do this, since the fry lose this open connection very early in life. As a result, the air bladder is often completely closed off (physoclistous) in fry that are more than a few days old, and they can no longer inflate their gas bladders this way.

Newborn seahorses are therefore physosymotous, and their gas bladders open into their esophagus via the pneumatic duct. If they are exposed to the air for any length of time in this condition, chances are great that they will swallow too much air and overinflate their gas bladders. When that happens, the fry develop fatal buoyancy problems and become the infamous "floaters" that bob helplessly at the surface until they starve to death. If you examine a floater carefully, you can actually see its overinflated swim bladder, which appears as a silvery bubble in its neck at the base of the throat.

In seahorses, the pneumatic duct closes off after a few days of development or less, depending on the species, and this open connection to the esophagus is lost. At that point, exposure to the air is no longer a threat to the physoclistous fry since they can no longer overinflate their gas bladders by gulping air.

I don’t have any data indicating at what age the pneumatic duct closes off in various species of seahorses, so I cannot answer your question much more precisely than that, Dave.

Interestingly, denying newborn seahorses access to the surface altogether immediately after birth can be just as harmful to them as lifting them out of the water and exposing them to the air. This is because newborns that miss the opportunity to gulp air at the surface while their pneumatic duct is still open — perhaps as the result of an oily or greasy film at the surface of the water — suffer from underdeveloped swim bladders. As they grow and become heavier, they sink to the bottom and are unable to swim or feed normally.

If denied access to the surface to inflate their swim bladders, the fry behave normally while they are small and their weight is still negligible. But over the weeks, as they grow and put on weight, their underdeveloped swim bladders and inability to achieve neutral buoyancy increasingly handicap them. Once they gain a little weight, they sink like rocks. Unable to swim, they are reduced to slithering along the bottom on their bellies and are commonly referred to as sliders. This deficiency does not become apparent until the fry are several weeks old. Needless to say, this hinders their swimming ability and severely limits their feeding opportunities, delaying their growth and development, and rendering entire broods useless. In several cases, the problem has apparently been traced back to an oily film on the surface of the nursery tank, which prevented the newborns from filling their swim bladders with air (Silveira, 2000). A protein skimmer will prevent this by removing filmy surface layers and surfactants in general.

Some breeders feel that "sliders" are many times the result of a genetic defect affecting the swim bladder, but whatever the cause and effect, the undeveloped or underinflated swimbladder is a crippling problem for the affected juveniles.

All of this creates a tricky situation for the seahorse keeper. Pelagic newborns that are unable to reach the surface and inflate their gas bladders may suffer from negative buoyancy and become sliders. On the other hand, newborn seahorses that are exposed to the air while being transferred from the tank they were born in to the nursery tank may swallow too much air and develop positive buoyancy as a result of their overinflated gas bladders, becoming floaters. It can sometimes be difficult to achieve the right balance between these two extremes because physosymotous fry can also accidentally ingest air while feeding on newly-hatched brine shrimp which tend to congregate at the surface where they are drawn by the light.

Ordinarily, pelagic seahorse fry will head for the surface to fill their air bladders immediately upon being born, often accomplishing this vital task before the seahorse keeper even realizes his or her expectant male has delivered, as previously described:

In other words, for all intents and purposes the home hobbyist need not be concerned about allowing his pelagic seahorse fry to fill their air bladders initially, which they do automatically, but should rather concentrate his efforts on preventing the newborns from accidentally ingesting too much air. By the time you noticed they have been born, their air bladders will have already been filled via the pneumatic duct, and they are ready to go in their kriesels/pseudokreisel nurseries.

In a small, closed-system aquarium problems with sliders are very uncommon, since the newborns always have free access to the surface, whereas problems with surface-hugging pelagic fry becoming floaters are almost universal. It is the latter that the home hobbyist must be concerned about and guard against.

Best of luck with your seahorses on their future progeny, Dave! Here’s hoping all will go smoothly and you’ll never have to be overly concerned about sliders or floaters.

Respectfully,
Pete Giwojna


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